In the manufacture of paper sheets by coating, there is deposited on the surface of the base paper an aqueous composition called a “coating dispersion” the function of which is to confer on the sheet a number of properties such as opacity, brightness, whiteness, printability by gravure or offset printing processes, etc.
These coating dispersions contain water, one or more mineral fillers such as natural or synthetic calcium carbonate, kaolin, talc, or again, titanium oxide, one or more binders of a natural origin (like the carbohydrates such as starch, casein, carboxymethylcellulose—
CMC), or of a synthetic origin (such as the styrene-butadiene, styrene-acrylic, vinyl copolymer latexes), as well as various additives (dispersants, water retention agents, optical brighteners, etc.).
Among these additives, there is a special category that allows the optimization of the rheological characteristics of the dispersion in relation to the coating process parameters and the properties sought for the dispersion: those of rheology modifiers. As indicated by their name, these products have the function of controlling the rheology of the dispersion, i.e. of adjusting its viscosity as a function of the stress that is applied to it.
In this regard, it is in the first place important to have an easily pumpable and filterable dispersion in the feed circuits of the coating process, with no tendency for the formation of foam or coating splash as well as a too rapid sedimentation. This requirement corresponds to an increase in the viscosity under low gradient shear, or a Brookfield™ viscosity measured at 100 RPM and at 25° C. with the device of the same name, without which the dispersion is too liquid.
Another important rheological characteristic is the viscosity under high shear gradient, as expressed by an ACAV viscosity at 25° C., measured in a capillary viscometer where the dispersion can be subjected to high shear gradients (from 105 to 3×106 s−1) of the same order of magnitude as those observed during the coating process in the application of the coating blade scraping off the excess dispersion deposited. The viscosity under high shear gradient is a determinant of the blade pressure to be applied. The higher the viscosity under high shear gradient, the higher the blade pressure must be to control the weight of the coat deposited.
However, the increase in the dry extract of coating dispersions and the increase in coating rates are trends that have been observed in recent years, because they have economic and/or quality advantages. Indeed, an increase in the dry extract of the coating dispersion permits a reduction in costs: the amount of energy needed for drying the dispersion is thus decreased. This increase also allows an improvement in the quality of the paper: the penetration of the dispersion into the base paper is reduced, which is favourable in the development of brightness. But this increase in dry extract leads to an increase in the viscosity under shear and as a consequence, to an increase the blade pressures required.
Higher coating speeds generate an increase in the hydraulic force on the blade and thus in the pressure to be exerted. Increases in blade pressure may then reach unacceptable levels and ones that are usually accompanied by dispersion overflows known by the terms “foams” or “beads”.
It is therefore useful for the person skilled in the art to have a rheology modifier in order to obtain the necessary low shear gradient viscosity and to reduce the viscosity under high gradient in order to exploit the benefits of an increase in speed or an increase in the dry extract in coating dispersions without exceeding the limits of his process and avoiding the formation of overflows. This problem is reported in document WO 84/04491. This double problem of increasing the Brookfield™ viscosity and reducing the ACAV viscosity can be summarized as a search for a so-called “shear-thinning” agent.
In parallel with these rheological aspects is another fundamental property of the coating dispersion: its water retention: After deposit on the base paper, the dispersion has a natural tendency to transfer part or all of the water-soluble substances it contains into base paper. An attempt is therefore made to reduce as much as possible the migration of water and water-soluble substances in order to prevent an evolution of the rheology of the unused coating dispersion that is recycled in the coating process. This is referred to as the phenomenon of water retention which one seeks to improve, i.e. to increase.
A special category of rheology modifiers has been known for decades, one which allows an increase in the Brookfield™ viscosity of coating dispersions while improving their water retention. These are the comb polymers with a (meth) acrylic skeleton with alkoxy polyalkylene glycol side chains with the following general formula:R—(AO)m—(BO)n—R′where:                m and n are integers that are less than or equal to 150, with at least one being a non-zero.        A and B designate alkyl groups that differ from one another and have 2 to 4 carbon atoms, group AO preferentially designating ethylene oxide and group BO preferentially designating propylene oxide.        R designates a polymerizable unsaturated function,        R′ designates a hydroxy or alkyl group with 1 to 5 carbon atoms.        
These structures are described in documents WO 01/96007 A1, WO 04/044022 A1, WO 04/041883 A1, WO 07/069037 A1 and WO 08/149226 A1, all incorporated herein by reference. These polymers can be introduced in the coating dispersion through the suspension of mineral substances in which they improve the rheology (WO 01/96007 A1). In addition to their ability to increase the Brookfield™ viscosity of the dispersion, they help to improve the optical azuration (WO 04/044022 A1) and brightness (WO 04/041883 A1) thereof They are also known to increase the water retention of the dispersion (WO 07/069037 A1). However, it is also known that they increase very significantly the viscosity under high shear gradient (WO 08/149226 A1) which is not compatible with a high speed and/or elevated dry extract coating.
In addition, these polymers appear as complex structures, in the sense that they are defined through multiple variables: the anionic species for the monomer forming the main chain, the possibility of implementing a termonomer and/or a cross-linking agent, and for the side chain, the nature of the polymerizable function R, the nature and number of alkyloxylated units, and finally, the identity of the terminal group (hydroxy or alkoxy with 1 to 5 carbon atoms). In addition, the polymers predominantly illustrated in the above documents are acrylic acid and methacrylic acid copolymers, with methoxy polyethylene glycol methacrylate.